Ferrate Preoxidation Alleviating Membrane Fouling through the Formation of a Hydrophilic Prefiltration Layer

Membrane fouling is one of the main obstacles impacting ultrafiltration (UF) membranes for water purification. Herein, it was found that a hydrophilic layer that is composed of a ferric floc formed in the reduction of ferrate coated on the membrane during the ultrafiltration of river water, and this layer is effective for attenuating membrane fouling. The newly formed ferric (hydro)oxide particles aggregated with natural organic matter (NOM) of the source water and then deposited on the membrane surface forming a negatively charged, porous, and hydrophilic prefiltration layer. This layer was beneficial for rejecting transphilic dissolved organic carbon (DOC), resulted in the accumulation of external membrane foulants, and alleviated internal membrane fouling. The membrane flux improved by 2.8 times, and the membrane fouling resistance decreased by 0.36 times after preoxidation with 1.5 mg/L of ferrate. Meanwhile, the performance of the membrane rejection was enhanced by this layer as well. DOC and UV254 were eliminated by 41% and 67% at the condition of 2.0 mg/L ferrate preoxidation coupled with UF. A model analysis suggested that intermediate and standard pore blocking were enhanced after ferrate preoxidation, while cake layer fouling was substantially attenuated. The ferric prefiltration layer can be back-washed for the recovery of membrane flux and be repeatedly generated through ferrate preoxidation. The combination of ferrate preoxidation with UF takes full advantage of the oxidizability of ferrate and the aggregation/adsorption potential of ferric (hydro)oxides by the reduction of ferrate.